Image processing devices such as copiers and printers are a convenient and simple means of producing multiple copies of documents which have the same content as the original. However, if an original which must not be made public, such as a top-secret document, is copied, it is conceivable that copies of it will find their way out into the world. As long as an original exists, it is very difficult to monitor or detect the fact that a copy of it exists or that the copy has been removed from the premises. This has prompted a demand for some way to make it impossible to copy critical papers such as top-secret documents. This has become increasingly true as trade secrets have become the focus of greater attention.
To meet this demand, devices have been developed to prevent the copying of proscribed documents. Such devices store an image associated with a specified pattern. They render a judgment as to whether an input image, which is the image meant to be copied, contains the specified pattern. If it does, they make the entire screen go black or prevent the copy operation from proceeding. If the paper used for confidential documents is imprinted with this specified pattern, then, they will, in theory at least, be impossible to copy.
Recent improvements in the quality of full-color copiers have made it nearly impossible to distinguish a copy from an original with the naked eye. In this case there are devices which allow the document to be copied but indicate on the copy that it is not an original. If the specified pattern is found in the input image, the copy will be output with a logo such as the word "copy" imprinted faintly on it.
In either case, it is necessary to detect whether the input image contains the specified pattern. To extract the features needed to detect the pattern, the image is binarized using peripheral contrast and the portion of the image containing the features is extracted.